Apparatus for controlling electric motors



June 1947. o. w. LIVINGSTON 2,421,632

AP PARATUS FOR CONTROLLING ELECTRIC MOTORS Filed June 16, 1945 4Sheets-Sheet 1 Pig. i'a.

Inventor: Orrin W Livingston,

Qy @a IS Attornqy.

June 3, 1947. 0, w, v 5 7 2,421,632

APPARATUS FOR CONTROLLING- ELECTRIC MOTORS Filed June 16, 1945 4Sheets-Sheet 2 Fig. lb.

25' Z5-\- a W /25c Inventor: Orrin W. Livingst'crm' is Attorney.

June 3, 1947. o. w. LIVINGSTON APPARATUS FOR CONTROLLING ELECTRIC.MOTORS I Filed June 15, 1945 4 Sheets-Sheet 3 Invehtor Orrin WLivingston,

as Attorney.

June 3, 1947. o. W. L|V|NG $TON APPARATUS FOR CONTROLLING ELECTRICMOTORS Filed June' 16, 1945 4 Sheets-Sheet 4 FORWARD MOTORING GENERATINGREVERSE A Q r GENERATING FORWARD MOTORING REVERSE Inventor: Orrin W.Livi ston,

I 7 His Attorney Patented June 3, 1947 UNITED STATES PATENT OFFICE OrrinW. Livingston, Scotia, N. Y., assignor to General Electric Company, acorporation of New York Application June l6,

22 Claims. 1

This invention relates to apparatus for controlling electric motors, andit has for an obiect the provision of a simple, reliable, and improvedcontrol apparatus of this character.

More particularly, the invention relates to motor control apparatus inwhich the armature is supplied from a single electric valve typerectifying equipment. Since a single control rectifying equipment canconduct current in only one direction, control apparatus of thischaracter has not been suitable for use with motors which in theiroperation may be required to produce a negative torque to brake anoverhauling load. Accordingly, a further object of this invention is theprovision of motor control apparatus in which the armature is suppliedfrom a single electric valve type rectifying equipment and in' which thetorque of the motor may be continuously varied from a maximum positivevalue to a maximum negative value without reversing the armaturecurrent.

A further object of the invention is the provision of a motor controlapparatus of this character in which the rectifier is controlled tooperate as an inverter to effect retardation of the motor when the motoris decelerated from a high speed to a lower speed, or when the motor isoverhauled by its load.

This application is a continuation in part of application Serial No.568,585 for Motor control apparatus, filed December 18, 1944, andassigned to the same assignee.

In carrying the invention into eilect in one form thereof, a singlecontrolled electric valve type rectifying equipment is provided forsupplying current to the armature of the motor. Regulating meansresponsive to the armature current are provided for controlling therectifier to maintain the armature current constant. A controlledelectric valve rectifier is provided for supplying current in onedirection to the field winding to cause the motor to produce a p sitivetorque, and a second controlled electric valve rectifier is provided forsupplying current in the reverse direction to the field winding to causethe motor to produce a negative torque.

Means are provided for producing a signal voltage which bears apredetermined relationship to some operating characteristic such as thevoltage supplied to the armature or to the field winding, the current inthe field winding, the speed of the motor, or the tension of a length ofmaterial upon which some operation is being performed. A source ofadjustable reference voltage is provided, and the signal voltage iscompared with 1945, Serial No. 599,911

(01. rza-zss) this reference voltage. Means responsive to the polarityof the difference of the signal and reference voltages are provided forselectively enengizing the forward and reverse field rectiflers and forcontrolling the energized rectifier in response to the magnitude of thedifference voltage.

For the purpose of increasing the torque for short periods of time, suchas during periods of acceleration and deceleration, means responsive tothe diflerence or the signal and reference voltages are provided forcontrolling the armature rectifier to force the armature current toexceed its rated value. Preferably, the control is arranged so that thearmature forcing action is not initiated until the field current hasattained full rated value.

The current regulator for the armature rectitor is provided; 1. e., itmay be considered to be r the case in which the current regulator isadjusted to hold zero armature current.

Suitable voltage and current limiting means may be provided forpreventing excessive armature voltages and currents.

For a better and more complete understanding of the invention, referenceshould now be had to the following specification and to the accompanyingdrawing of which Fig. 1 is a simple, diagrammatic sketch of anembodiment of the invention; Figs. 2 and 3 are simple, schematicdiagrams of modifications; and Figs. 4 and 5 are charts ofcharacteristic curves which serve to facilitate an understanding of theinvention.

Referring now to the drawing, an electric motor I having an armature laand a shunt field winding lb is supplied from a source of alternatingvoltage 2 through a supply transformer 3 and a suitable controlledrectifying equipment.

The speed and direction of rotation of the motor are under the controlof a suitable controlling accessory which is illustrated as a speedcontrolling rheostat 4. A push-button control station for controllingthe starting and stopping of the motor is optional equipment. However,since such push-button control stations are conventional and since thestarting and stopping of the motor may be eflected by the speedcontrolling rheostat, such push-button control station is omitted fromthe drawing in interest of simplicity.

The secondary winding 3a of the supply transformer 3 is provided with acenter tap 3b and this center tap is connected to the bus I whichtherefore becomes the negative terminal of the direct current system, i.e., the negative armature and the negative field terminal.

The current supplied to the armature Ia of the motor is controlled bythe single biphase halfwaverectifying equipment which comprises the twoelectric valves 6 and I. This rectifier rectifies both halves of thealternating voltage wave. The anodes 8a and 1a are connected through theprimary windings la and Ibof a special control transformer 8 to theopposite terminals of the secondary winding of the supply transformer 3.The cathodes 9b and lb of the valves 6 and I are connected to theconductor 8 which thus becomes the positive side of the supply for thearmature. Thus the armature circuit is readily traced from the positiveconductor 9 through the armature is to the negative conductor 5.

The current supplied to the field winding lb from the source I iscontrolled by two single rectifying equipments. The first rectifyingequipment which controls the current supplied to the field winding Ib toproduce a positive torque, i. e., a torque which produces rotation ofthe motor in the forward direction, comprises the two electric valvesIll and II. The rectifier which controls the supply of current in thereverse direction to produce a negative torque, i. e., a, torque whichproduces rotation in the reverse direction, comprises the two electricvalves l2 and I3. As

shown, the anodes Illa and Ila of the valves I and II are connected tothe opposite terminals of the secondary winding 3a of the supplytransformer 3. The cathodes lb and Iib of these valves are connected tothe conductor I4 which thus becomes the positive terminal of the fieldsupply for the forward direction of rotation. Theanodes I20. and lid ofthe reverse rectifier for the field are connected to the conductor I5which thereupon becomes the nega. tive terminal of the field supply forthe reverse direction of rotation of the motor. The cathodes IZb and I3bare connected to the opposite terminals of the secondary winding of thesupply transformer so that the conductor 5 becomes the positive bus ofthe field supply for the reverse direction of rotation of the motor.

Although the electric valves 8, 'I, III, II and I2, I3 may be of anysuitable type, they are preferably grid controlled, mercury vaporthyratron tubes. Their cathodes may be either of the directly heated orindirectly heated type. Preferably, they are of the indirectly heatedtype as illustrated, and are provided with suitable heating units (notshown). These thyratrons i, I, I0, II, I2 and I3 are provided withcontrol grids 60, 10, I00, IIc, I20 and I3c, respectively. In thyratronvalves, the function of the control grid is only to initiate the flow ofcurrent between the anode and cathode during each positive halfcycle ofanode voltage. Once current has started to flow, the grid exercises nofurther control until the conductivity of the valve has been interruptedby some means external to the valve itself. Once the current has ceasedto fiow, the potential of the grid will again determine the point in thepositive half-cycle of anode voltage at which the valve will againbecome conducting.

' 4 These valves are therefore grid controlled arc rectifiers.

Although the thyratron valves I, l, ll, II, II, II may be controlled byany suitable method, it is preferred to use the method of varying thefiring point of the valve during the positive halfcycle of anodevoltage. For the carrying-out of this method of control, three phaseshifting networks, one for the armature thyratrons I and I, and one eachfor the forward and reverse field thyratrons are provided. The phaseshifting network for the armature thyratrons comprises a resistor I8 andthe alternating current winding IIa of a saturable core type reactor II.The network for the forward field thyratrons comprises a resistor I8 andthe alternating current winding lfla Of a saturable core type reactorII. Similarly, the network for the reverse field thyratrons comprises aresistor 20 and the alternating current winding Ila of a saturable coretype reactor II. These phase shifting networks are connected in parallelacross the terminals of the secondary winding 22 of a low voltagetransformer of which the primary winding is supplied from a suitablesource such as that represented by the supply line 2. The secondarywinding is provided with a center tap 22a. The primary winding 23a of agrid control transformer H is connected between the center tap 12a andthe junction point I is of the resistor l6 and the reactor winding IlaThe secondary winding of this grid transformer is divided into twohalves 21b and c. The half bis connected between the grid and cathode ofthe armature thyratron l, and similarly, the half 230 is connectedbetween the grid and cathode oi the armature thyratron I. Correspondinggrid transformers 24 and II are provided for the forward and reversefield thyatrons. The field transformer 24 has a primary winding which isconnected between the center tap 22a and the junction point Ila of theresistor II and reactor winding I941. The secondary winding is dividedinto two halves 24b and c which are connected between the cathodes andgrids of the forward field thyratrons I0 and II, respectively. The gridtransformer 28 has a primary winding 250. which is connected between thecenter tap 22a and the Junction point "a of the resistor 20 and reactorwinding Zia. This grid transformer has two secondary windings "b and 250which are connected between the cathodes and grids of the reverse fieldthyratrons I2 and I3, respectively. The phase shift of the grid voltagesis produced by varying the reactance of the saturable core reactors,which is controlled by varying the D.-C. saturation of these reactors.

The control is such that when the saturable reactors are saturated, thevoltages of the grid transformers tend to be in phase with the anodetransformer voltage, and when the reactors are unsaturated, the voltagesof the grid transformers tend to be out of phase and lagging.Intermediate values of saturation produce corresponding intermediatephase relationships. Thus, when the saturable reactors I1, I! and ii arefully saturated, the thyratrons 6, I, III, II, II and II are fullyconducting, and conversely, when the reactors are unsaturated, thethyratrons are nonconducting. For intermediate values of saturation, thethyratrons have corresponding intermediate values of conductivity.

A direct control voltage is derived from the alternating voltage source2 by means of a small auxiliary biphase half-wave rectifying valve 21 ofwhich the anodes 21a and 21b are connected to opposite terminals of thesecondary winding 3 v of a transformer 28, of which the primary wind- 32and 33.v These glow tubes 32 and 33 are gase- I ous discharge deviceswhich operate in that region of their characteristics in which thevoltage drop across the tube is substantially constant over a wide rangeof current. The voltage drop across the points 3|a and 33a is fixed inmagnitude by the type of glow tube used, and within the operating limitsof this equipment, this voltage is independent of variations in thealternating current supply voltage. Any difference in voltage betweenthe voltage across the capacitor 30 and the constant voltage across theglow tubes 32 and 33 is absorbed by the resistor 3|. The constantvoltages across the glow tubes 32 and 33 are used for stabilizing thevoltage on the amplifier valves used in the control circuit, and inaddition, reference voltages which are used for controllin purposes arealso derived from the sum of these voltages.

For the purpose of varying the direct current which fiows in thesaturating winding MD, a suitable amplifying electric valve 34 isprovided. This valve is provided with an anode 34a, a cathode 34b, and acontrol grid 340. The D.-C. winding llb of the saturable reactor and thevalve 34 are connected in series with a resistor-35 across the glow tube32. The control of the current through the D.-C. winding of the armaturesaturable reactor I1 is achieved by proper choice of the grid-to-cathodevoltage of the valve 34. As the voltage of the grid 340 is made lessnegative with respect to the voltage of the cathode 341), the currenttransmitted by the valve increases, thereby increasing the saturation ofthe armature saturable reactor I! which, as pointed out in theforegoing, results in increasing the voltage applied to the armature ofthe motor I. Conversely, as the voltage of the grid 340 is made morenegative with respect to the voltage of the cathode 34b, the currenttransmitted by the valve 34 decreases and this decreases the voltagesupplied to the armature of the motor.

For the purpose of regulating the armature current, 1. e., maintainingthe armature current constant at the rated value or some desiredfraction thereof, means are provided for comparing a signal voltagederived from the anode current of the armature thyratrons with areference voltage, and utilizing the difference of these signal andreference voltages to control the armature thyratrons in such a manneras to maintain the armature current constant at the desired value. Thesemeans are illustrated as comprising the anode current transformer 8, thebiphase rectifying electric valve 36, and a filter comprising thereactance 31 and capacitor 38. As shown, the two primary windings 8a and8b of the anode current transformer 8 are connected in series with theanode circuits of each of the armature thyratron valves, and thistransformer is polarized in such a manner that when one of the armaturethyratrons conducts, the fiux in the core is in one direction, and whenthe other thyratron conducts, the flux is reversed. As a result, analternating voltage is induced across the secondary winding 30, and themagnitude of this induced voltage is determined by the resistance loadconnected between the primary 6 and secondary windings. This alternatingvoltage is rectified by the electric valve 33 and appears as a directvoltage across the resistor 33 and the conductors 40 and 4|. Theconductor 4|), it will be noted, is connected to the junction point 32aof the glow tubes 32 and 33.

An adjustable reference voltage is derived from the voltage across theglow tube 32 by means of a potentiometer 42 which is connected betweenthe conductors 40 and 43, which are connected across the terminals ofthe glow tube 32. The grid 34c of the armature current regulatingcontrol valve 34 is connected to an intermediate point on a voltagedivider which is connected between the slider 42a and the conductor 4|.As aresult. of these connections, the armature current signal voltage iscompared ,with the reference voltage between the conductor 40 and theslider 42a, and the difference is applied between the cathode 34b andthe grid 340 of valve 34.

If the armature current decreases below the value at which it is to beheld constant, the current signal voltage decreases correspondingly andthe voltage of the grid 34c becomes more positive. As a result, theregulating valve 34 conducts an increased current, thereby increasingthe current in the D.-C. winding of the saturable reactor IL Thisreduces the reactance of the 'reactanc winding Ila and advances thephase of the grid excitation of the armature thyratrons 6 and I so thatthe current is restored to the desired constant value. If the armaturecurrent increases above the desired constant value, the reverse actiontakes place. The value at which the armature current is maintainedconstant may be varied throughout a wide range of values between zeroand rated armature current by adjustment of the position of the slider42a.

For the purpose of varying the saturating current of the forward andreverse field saturable reactors l3 and 2|, there is provided atwo-stage electric valve amplifier of which the first stage comprisesthe valves 44 and 45 and the second stage comprises the valves 46 and41. The first stage valves 44 and 45 are connected across the constantpotential buses 43 and 48. Substantially equal voltage droppingresistors 49 and 50 are connected in the anode circuits of both valves,and a resistor of rather high ohmic value is common to the cathodecircuits of :both valves. This resistor is illustrated as comprising afixed resistance portion 5| and an adjustable resistance portion 52.Preferably, the two valves 44 and 45 are matched and are contained in asingle envelope. The control grid of the valve 45 is connected to theconductor 40, and the control grid 'of the valve 44 is connected to anintermediate point on a voltage dividercomprising two resistors 53 and54 connected in series between the slider 4a of the control rheostat 4and the terminal 55 which constitutes one side of a source of signalcontrol voltage.

The second stage valves 46 and 41 are respectively connected in serieswith the saturating windings |9b and 2|b of the forward and reversesaturable reactors across the constant potential buses 43 and 40. Thecontrol grid of the valve 43 is connected to an intermediate point of avoltage divider comprising the resistors 56 and 53a which is connectedbetween the anode 44a of valve 44 and the negative side 48 of theconstant voltage source. Similarly, the control grid to the secondarywinding and by the turn ratio of the valve 41 is connected to anintermediate point of a voltage divider comprising the resistors 51 and58 which is connected between the anode 45a of the valve 45 and thenegative side of the source 48. I

The rheostat 52 is adjusted so that when the slider 4a of the controlrheostat 4 is in the central or zero position, both valves 44 and 4! areconducting substantially equal currents of which the value isapproximately at the middle of the useful range, i. e., the linearportion of the range. Owing to the.voltage drop across the resistors 48and 58 produced by the current conducted by valves 44 and 45, thevoltages applied to the control grids of the valves 48 and 41 are suchthat these valves conduct very small amounts of current, i. e.,approximately five per cent of the amount of current required tosaturate the reactors l9 and 2|. This "overlap of the valves 48 and 41can be varied as desired by adjusting the rheostat 52. When bothreactors l8 and II are substantially unsaturated, the grid voltages ofthe forward and reverse field thyratrons are retarded wtih respect tothe anode voltages, and either zero current or a very small circulatincurrent flows in the field winding thyratron circuit.

If the voltage applied to the grid of the valve 44 is made more positivewith respect to the voltage of its cathode, the valve 44 conducts anincreased current, and this increased current produces an increasedvoltage drop across the resistors 5i and 52. This increased voltage dropmakes the voltage of the cathode of the valve 45 more positive withrespect to the voltage of its grid than the voltage of the cathode ofvalve 44 is with respect to the voltage of its grid owing to the factthat the voltage of the grid of valve 46 does not change. Consequently,the current conducted by the valve 45 is decreased by substantially thsame amount that the current in the valve 44 is increased. The increasedcurrent conducted by the valve 44 produces an increased voltage dropacross the voltage dropping resistor 48, and this results in decreasingthe voltage supplied to the grid of the valve 45. Likewise, thedecreased current conducted by the valve '45 results in decreasing thevoltage drop across the resistor 58, and the result of this is toincrease the voltage supplied to the grid of the valve 41. The decreasein the voltage supplied to the grid of valve 46 reduces the conductivityof the valve substantially to cutoff, and the increased voltage suppliedto the rid of th valve 41 causes the valve to conduct an increasedamount of current, thereby increasing the saturation of the reversefield saturable reactor 2|. The result is that the forward fieldthyratrons i0 and I I are rendered nonconducting and the reverse fieldthyratrons l2 and I3 conduct an amount of current which is proportionalto the positive increase in voltage applied to the grid of the valve 44.

If the voltage supplied to the grid of the valve 44 is decreased insteadof increased, i. e., made more negativ with respect to the voltage ofits cathode, the reverse action takes place. That is to say, theconductivity of the valve 41 is reduced to cutoff, and the valve 44conducts an increased amount of current thereby to saturate the forwardreactor a corresponding amount and advance the phase of the voltagesupplied to the grids of the forward field thyratrons. This causes theforward field thyratrons I8 and ii to supply a current in the forwarddirection to the field winding lb. The value of this current correspondsto the amount by which the voltage of the grid of valve 44 was made morenegative with respect to the voltage of its cathode. The characteristicwhich is to be maintained constant is determined by the signal voltagethat is supplied between the conductor 48 and the terminal 55. Forexample, if it is desired to maintain the torque of the motor Iconstant, a signal voltage derived from the field current by means of avoltage dropping resistor connected in the circuit of the field windingi supplied between the conductor 4|! and the terminal II. If it isdesired to maintain the horsepower output of the motor constant, asignal voltage derived from the armature voltage of the motor issupplied between the conductor 40 and the terminal 55. Likewise, if itis desired to maintain the speed of the motor constant, a signal Voltageproportional to motor speed is supplied between the conductor 48 and theterminal II.

In the modification of Figs. la, 1b, a voltage proportional to the speedof the motor I is supplied between the conductor 40 andterminal I! bysuitable means such as a tachometer generator 59 which is driven by themotor I. The armature connections of this tachometer generator are suchthat if the motor I is rotating in the forward direction, the negativebrush of the tachometer generator is connected to the conductor 40 andthe positive brush is connected to the terminal 55. When the motor i isrotating in the reverse direction, the polarity of these brushes isreversed.

Under certain conditions of operation, it is found desirable andpractical to increase the torque of the motor for short periods of timeby permitting the armature current to exceed its rated value. This isparticularly the case where rapid acceleration or deceleration isrequired. To provide this feature, electric valves 88, 8| and 62 areprovided together with potentiometers I8, 64, and 66 and necessaryresistors. For the purpose of limiting the armature current to apredetermined maximum safe value, electric valves 61 and 68 are alsoprovided together with a potentiometer 69 and necessary resistors.

The two valves 60 and ii are connected in parallel with the regulatingvalve 84 in circuit with the saturatin winding Ilb of the armaturesaturable reactor. The grid of the valve II is connected to the slider83a of rheostat 83 which is connected across the constant potentialbuses 43 and 48. This grid is also connected through a resistor H and aconductor 12 to the intermediate point of the voltage divider to whichthe grid of the field regulating control valve 44 is connected. The gridof the valve 8| is connected to the slider 84:; of rheostat 64 which isconnected between the anode of valve 82 and the negative bus 48. Thegrid of the valve 42 is connected to the same intermediate point of thevoltage divider 53, 54 to which th grid of valve ill is connected. Thevalve 82 is connected across the buses 43 and 48 and a voltage droppingresistor 13 is connected in its anode circuit. The grid bias voltages ofthe valves 80, GI and 82 are so adjusted that when the motor is at restor dur ing normal steady-state running conditions, the valves 68 and 6|are nonconducting and the valve 62 is fully conducting, and thiscondition continues unless the voltage of the intermediate point 54achanges by more than the amount that is required to turn either theforward or the reverse field thyratrons full on. If the voltage of thepoint 54a becomes more positive than the amount required to turn thereverse field thyratrons l2 and II full on, the valve 60 is renderedconducting and the resulting increased current through the saturatingwinding Ill) the armature saturable reactor advances the phase of thegrid voltage of the armature thyratrons 6 and 1, thereby increasing thecurrent supplied to the armature of the motor. However, since the valve82 is fully conducting, the positive increase in voltage of the point54a does not produce any substantial change in its conductivity. Thevoltage drop across the resistor 13 maintains the voltage of the grid ofvalve Ii sufficiently negative with respect to its cathode to preventconduction.

Likewise, it the voltage of the intermediate point 54a changes to becomemore negative by a greater amount than is required to turn the forwardfield thyratons l0 and II full on, the voltage of the grid of valve 82becomes more negative with respect to the voltage of its cathode,thereby decreasing the current flow through the tube. This decreasedcurrent flow reduces the voltage drop across the resistor I3, therebymaking the voltage of the grid of valve 6i more positive with respect toits cathode. As a result, the valve BI is rendered conducting and theresulting increased current flow through the saturating winding Nb ofthe armature saturable reactor advances the phase of the grid voltage ofthe armature thyratons, thereby increasing the current supplied to thearmature of the motor i. Thus, irrespective of whether the voltage ofthe intermediate point 54a changes in a positive sense or in a negativesense by more than the amount required to turn either the forward or thereverse field thyratons full on, one or the other of the valves 60 orBI. is rendered conducting, thereby to advance the phase of the gridvoltage of the armature thyratons and to increase the current suppliedto the armature.

If during the accelerating period, the armature current should tend tobecome excessively high so as to endanger commutation, the currentlimiting valves 61 and 68 ar brought into action. As the armaturecurrent increases, the armature current signal voltage increases,thereby lowering the voltage of the normally rather positive potentialof the slider 69a of the rheostat 69. If the armature current increasessufllclently, the voltage of this slider which is connected to thecathodes of the diode valves 51 and 68 falls below the grid voltage ofwhichever of the valves 60 or BI is conducting and producing the forcingaction. This causes the one of the diode valves 61 or 88 which isconnected to the grid of whichever of the forcing valves 60 or BI isactive to become conducting. This produces an increased voltage dropacross either of the resistors H or 14, thereby lowering the gridvoltage of whichever of the valves 60 or BI is producing the forcingaction. This then limits the forcing ability of the valves 60 or 6! to avalue of armature current the magnitude of which is determined by thesetting of the slider of the potentiometer 59.

I! the motor is to be operated above'its basic speed. i. e., its speedwhen driving rated load at rated field current, means are provided forlimiting the field excitation so as to limit the armature voltage to asafe value. This means is illustrated as an electric valve T of themagnetron type excited by means of a potential coil 14 connected acrossthe motor armature terminals, together with a pair of diode controlvalves abscissae represent torque.

' l1 and 18 controlled by the magnetron valve 15.

The anodes of the diode control valves 11 and I! are respectivelyconnected to the control grids of the second stage forward'and reversefield control valves 46 and 41. The cathodes of the control ,valves 11and i=8 are connected to the slider 19a of a rheostat l8 which isconnected in series with the magnetron valve across the constantpotential buses 43 and 48. As long as the armature voltage of the motoris below the desired value, the ampere turns in the magnetron coil 18are so low that the magnetron valve is fully conducting. This causesthe-slider 19a of the rheostat I9 and, consequently, the cathodes of therstraining diodes 11 and 18 to be higher in potential than the controlgrids of the forward and reverse field control valves 46 and 41.Consequently, an inverse voltage appears across the control diode valves11 and 18 and the normal control of the second stage control valves 48and 41 is unaflected. However, if the armature voltage is increased ineither direction, the magnetron impedance is effectively increased andthe voltage drop across the magnetron is correspondingly increased. Thiscauses the voltage of the slider 19a to decrease, i. e., become morenegative until the cathode voltages of the restraining diode valves 11and it become more negative than their anode voltages. In consequence ofthis the diodes l1 and T6 become conducting thereby to produce voltagedrops across resistors 46a and 41a and to increase the voltage dropsacross the voltage dropping resistors 56 and 51. This increased voltagedrop makes the grids of the second stage valves 48 and 41 more negative,thereby reducing the excitation of the field and consequently reducingthe armature voltage. Whichever of the second stage triode valves 46 or41 isconducting and producing the excessive field will feel therestraining effect of the diode valves first, since the voltage of itsgrid is more positive than the voltage of the grid of the valve which isnonconducting. One of the advantages of the use of the magnetron valve15 in the voltage limiting circuit is that the magnetic coupling schemeinsulates the control circuits from the power circuit.

,In Fig. 4, the relationships between speed and torque for variousarmature voltages with the armature current always in the same directionand held constant at rated value are represented by curves of whichordinates represent speed and ordinates above the zero axis representspeed in the forward direction and ordinates below the zero axisrepresent speed in the reverse direction. Abscissae to the right of thezero axis represent positive torque and abscissae to the left of thezero axis represent negative torque. The curves '80, 8! and :82 in thefirst quadrant represent the speed-torque relationships for motoringoperation in thefor: ward direction at 100 per cent, 50 per cent, and 25per cent armature voltages, respectively. Similarly, the curves 83, 84and 85 in the third quadrant represent the speed-torque relationshipsfor motoring operation in the reverse direction at per cent, 50 percent, and 25 per cent armature voltages. The curves 86, 81 and '88 inthe second quadrant represent the speed-torque relationships forgenerating action in the forward direction at 100 per cent, 50 per cent,and 25 per cent armature voltages, respectively, and the curves 89, 80and, 9| in the fourth quadrant represent the same relationships forgenerating action in the reverse direction.

If constant rated armature current is allowed to flow and the fieldsuitably excited so that the field flux is zero, zero torque will beproduced, and if the load has no inclination to turn of its own accord,zero speed results. If the field flux is axfiusted to a low positivevalue, a small positive torque is produced which tendsto cause rotationin the forward direction'until the torque of the load equals thatproduced by the motor. If a greater torque is desired, the field may bestrengthened until the desired torque is obtained. If the motor isconnected to a load device which .succeeds in turning the motor in thereverse direction, it resists with a, positive torque. As the directionof rotation of the armature reverses, the armature counter voltagereverses so that it has the same polarity as the voltage supplied by thearmature thyratrons. As the reverse-polarity counter voltage increases,the rectifier voltage decreases to zero and increases in the reversesense. Consequently, power is pumped back into the constant currentsource. Actually during this mode of operation, the motor is functioningas a generator.

If the field is excited in the reverse direction, an opposite ornegative torque is produced in the motor which causes the motor tooperate in the reverse direction. As shown by the curves in the secondand third quadrants, it will produce torque at any speed, positive ornegative, within the limitations of the motor. Thus it will be seen thatsuch a system is capable of supplying any desired torque, positive ornegative, or any speed, positive or negative, within the ratings of thethyratrons without a reversal of armature current.

With the foregoing understanding of the elements and their organization,the operation of the system will readily be understood from thefollowing detailed description. Assuming that the slider 42a is in itszero or extreme counterclockwise position and the slider la is in itscentral or zero position, the motor i will be at rest with approximatelyzero current in the armature and in the field winding. If it is desiredto operate the motor with rated armature current, th slider 42a is movedto the IOU-per cent position. As a result, the valve 34 is renderedfully conducting and the phase of th'e grid voltage of the armaturethyratrons is advanced. As the current supplied to the armature by thethyratrons increases, the current signal voltage across the conductors land H increases, thereby rendering the voltage of the grid 0 lesspositive. A balanced condition is established when the armature voltageattains a value such that the difference between the reference voltageat the slider 42a and the armature current signal voltage is justsufiicient to cause the armature thyratrons to conduct rated armaturecurrent. Any change in the armature current from rated value produces achange in the difference between the reference voltage and the currentsignal voltage of such extent as to effect the voltage of the grid a torestore the armature current to rated value.

To effect rotation of the motor in th'e forward direction, the slider lais moved in a counterclockwise direction from its zero position. As aresult, the voltage of the grid of valve 44 is made more negative,thereby decreasing the conductivity of valve 44 and increasing theconductivity of valve. This results in turning on the sec- 0nd stagetriode valve 46 and turning of! the second stage triode valve 41. Thephase of the grid voltage of the forward field thyratrons is therebyadvanced and current is supplied to the field winding lb of the motor ina direction to affect rotation of the motor in the forward direc- Sincethe change in the voltage of. the intermediate point a is sufficient tocause the forward field thyratron to supply full field current to thefield winding ib, it is sufficient to decrease the conductivity of the"sense" inverting valve II, thereby rendering the forward armatureforcing control valve Oi conducting. This initiates the armature currentforcing action described in the foregoing. Accordingly, the initial rateof acceleration is high.

As the speed of th'e motor increases, the speed signal voltage of thetachometer generator It increases correspondingly, thereby diminishingthe difference between the signal voltage and the reference voltage andmaking the voltage of the point "a more positive. This results in firstdiminishing the forcing action of the armature thyratrons until thearmature current is reduced to the value which the regulating valve 34is set to hold, and then decreasing the current supplied by the forwardfield thyratrons to the field winding i b. The acceleration of the motorcontinues but at a decreasing rate until at a speed corresponding to theposition of the slider la, a balanced condition is established such thatany further increase in the speed of the motor would diminish thedifference between the signal and reference voltages to such an extentthat deceleration of the motor would result. The motor continues tooperate at this preset speed as long as the load remains constant.

If, for any reason, the load should increase substantially, the speed ofthe motor would decrease slightly. This decreased speed increases thediflerence between the signal and reference voltages and this results inincreasing the current supplied to the field winding and againincreasing the armature current to re-establish the forcing action. Themotor accelerates until the balanced condition is re-established at aspeed just enough lower than the original speed to produce suilicientdifference between the signal and reference voltages to cause theforward field thyratrons to supply the additional amount of fieldcurrent necessary to produce the increased torque required by theincreased load.

For a decrease in load, the action is the reverse of that described inthe immediate foregoing.

In the event that the load becomes overhauling, the speed of the motorrises and the voltage of the tachometer generator increases sufiicientlyto turn off the forward field thyratrons l0 and ii and to turn on thereverse field thyratrons i2 and i3. This resulting reversal of the fieldof the motor first decreases the counter voltage of the motor and thenincreases it in the reverse direction. As a result, the armature currenttends to increase, but this is inhibited by the action of the currentregulator valve N which responds to the increased armature currentsignal voltage to retard the phase of the grid voltage of the armaturethyratrons to maintain the current substantially constant. The reversalof the counter voltage of the motor, the inductance of the armaturecircuit and the retardation of the phase of the grid voltage combine toproduce inverter operation of the armature thyratrons which produces avigorous braking action of the motor and returns power to the source.

The speed torque relationship for the motor when under the control ofthe armature current and speed regulating equipment of Figs. 1a and 1bis represented graphically by the curve 92 of Fig. 5 in which ordinatesrepresent speed and abscissae represent torque. The portion of the curve92 at the right of the zero vertical axis represents the motoringoperation. and the portion at the left represents the regenerativebraking operation resulting from the inverter operation in response toan overhauling load. The characteristic rises as the load is decreased,and continues to rise with increasing overhauling load in theregenerative braking quadrant. The rise is steepest at the point ofcrossing the zero vertical axis, 1. e., the point at which the operationchanges from motoring to regeneration.

In some applications this rising characteristic is undesirable. For thepurpose of modifying the characteristic as desired, a fadback circuitfrom the voltage across the field of the motor to the grid of one of theswitching valves 44 and 45 is-provided. This feedback circuit comprisesa resistor 93 and a capacitor 94 connected in series relationship acrossthe field winding lb. A potentiometer 95 is connected across thecapacitor. The resistor 93 and capacitor 94 constitute a filter acrossthe motor field so that the voltage developed across the potentiometer95 is a relatively smooth direct voltage. Any desired portion of thisvoltage is then selected by movement of the slider 95a and is fedthrough a series resistor 86 to the grid of one or the other of theswitching valves 44 or v45. Assuming that it is desired that the motorshall operate on a substantially flat speed torque characteristic suchas is represented by the curve 01 in Fig. 5, the voltage derived fromthe potentiometer 95 is fed to the grid of the valve 45. This produces aregenerative feedback action which controls the field thyratrons furtherto strengthen the field in response to a tendency of the motor speed todecrease during motoring operation and also to strengthen the field inresponse to a tendency of the motor speed to rise during regenerativeoperation. As a result, the speed torque characteristic is modified inaccordance with curve 91. If a more pronounced modification is desired,the slider 95a of the potentiometer 95 may be adjusted to vary theamount of the voltage fed back. Thus the characteristic may be modifiedso that the speed actually decreases with decreasing load in themotoring quadrant and also decreases with increasing overhauling load inthe regenerated braking quadrant as represented by the curve 98 in Fig.5. If it is desired to modify the characteristic in the opposite sense,i. e., to produce a more steeply decreasing speed with increasing loadin the motoring quadrant and an increasing speed with increasing load inthe regenerative quadrant, this may be accomplished by feeding back thevoltage from the field of the motor to the grid of the switching valve44. This results in a degenerative feedback and has the opposite effecton the speed torque characteristic as illustrated by the curve 99 ofFig. 5. It will be noted that the abrupt rise in the characteristicduring the transition from motoring to regenerative operation which isrepresented between the points 92a and 92b is absent from the modifiedcharacteristics represented by the curves 91, 98 and 99. This is a verydesirable feature.

If the slider 4a is suddenly moved to a lower speed position, i. e.,in'a clockwise direction toward the central zero position, the voltageof the intermediate point 54a becomes more positive,

thereby increasing the current conducted by the first switching valve 44and decreasing the current conducted by the second switching valve 45.This action turns the second stage reverse field valve 41 full on andturns on the second stage forward field valve 48. This results inrendering the forward field thyratron nonconducting and the reversefield thyratron fully conducting. The reversal of the field currentreverses the polarity of the motor voltage so that it adds to thevoltage of the armature rectifier and operates as a generator to returnpower to the source. Since the voltage of the point 541: is sufilcientlymore positive to turn the reverse field thyratrons full on, the forcingcontrol valve 00 is rendered conducting, thereby to increase theconductivity of the armature thyratrons and to increase the armaturecurrent above the value which the regulating valve 34 is set to hold. Asa result of the increased armature current and the generator operationof the motor I in returning power to the source, a vigorous negativetorque is produced which quickly brakes the motor to the lower speedwhich corresponds to the new setting of the slider 4a of thepotentiometer 4 without reversing the direction of the armature current.At this lower speed, the balanced condition is re-established.

If it is desired to cause the motor to rotate in the reverse direction,the slider is moved in a clockwise direction to a position on thereverse side of the potentiometer which corresponds to the desired speedin the reverse direction. The

reversing operation is the same as that described in the immediateforegoing except that the balanced condition is not re-established untilthe reverse or negative torque produced by the re.. verse field hasdecelerated the motor to rest and accelerated it in the reversedirection to the speed corresponding to the new position of the slider4a. This reversal or plugging operation is acconiplished withoutreversing the direction of the armature current. During a decelerationto stand-still the motor operates as a generator returning energy to thesource, thereby producing a vigorous braking torque which rapidlydecelerates the motor.

In the modification of Fig. 2, the armature forcing control portion ofthe apparatus, i. e., the portion within the dotted rectangle Hi0differs from the corresponding portion of the appara.. tus within thedotted rectangle IOI of the Fig. 1a, 1b modification. The maindifference in. the two modifications is that in the Fig. 1 modificationthe armature forcing control valves 60 and N are directly controlled inresponse to the voltage of the intermediate point54a in the inputcircuit of the switching valves 44 and which constitute the first stageof the field control amplifier, whereas in the Fig. 2 modification thecorresponding armature forcing control valves I02 and I03 derive theircontrol from the output circuits of the corresponding switching controlvalves I04 and I05 which constitute the first stage of the fieldcontrolled amplifier. In this connection, the grid of the forwardforcing control valve I02 is directly connected to the anode circuit ofthe forward field control switching valve I04, and the grid of thereverse forcing control valve I03 is connected to the anode circuit ofthe reverse field control switching valve I05. This connection makes itpossible in the Fig. 2 modification to eliminate the "phase inverting orsense inverting valve 62 of Fig. 1, since in the Fig. 2 modification thefunction of the 2,4a1,osa

sense inverting valve is performed by the for-ward neld switching valveI04. is that the modification of Fig. 2 permits a simpier adjustment ofthe "forcing takeover point owing to the amplified voltage change in theoutput circuit of the valves I04 and I05. Except for the performance ofthe functions of the sense inverting valve 82 of Fig. 1 by the valve 08,the operation of the Fig. 2 modification is essentially the same as theoperation of the Fig. 1 modification.

The modification of Fig. 3 is identical with the modification of Fig. 2with the exception that the armature current regulating control valveI06 and its associated control circuit, 1. e., the armature currentpresetting potentiometer I01 and the voltage divider resistors I08 andI09 are omitted.

The operation is the same as the operation of the Fig. 2 modificationwith the slider I010. of the armature current presetting potentiometerI01 in the zero armature current position. This will readily beunderstood from the following explanation.

From the description of the Fig. l and Fig. 2 modifications it is seenthat the armature forcing control feature may be used to obtainadditional motor output, not only for accelerating purposes, but also tosupply a sustained load which requires a torque greater than thatfurnished by the normal constant current limit setting at full fieldflux. This may require slightly greater speed regulation, since theincreased armature current is available only after sufilcient differencein the signal and reference voltages has been produced to produce fullcorrective field. However, with valve amplifying circuits very highdegrees of correction for very slight errors are easily obtained. Thusthe additional error is not excessive.

From a consideration of this it is evident that it is not necessary toset the slider of the constant current regulating potentiometer in theFig. 1 and Fig. 2 modifications at full rated armature current in orderto get full output of the motor. On the contrary, it may be set at the50 per cent, 25 per cent, or 10 per cent rated armature voltage. As lonas the torque required by the load is within the ability of the motor atthe set armature current and full field, the field will be held at asuitable value to maintain the required torque. If more torque isrequired, the field will attain its full value and the armature currentwill then be increased by the forcing action sufliciently to produce therequired torque.

Consequently, it is possible to set the constant current potentiometerslider for zero so that the armature current is controlled entirely bythe forcing control feature, As pointed out, this is obtained simply byomitting the normal constant current regulating valve 99 and itsassociated controi circuit entirely, thus further simplifying theequipment. An advantage of this modification is that most all of thedesirable features of the Fig. l and Fig. 2 modifications are retainedand in addition, the motor is operated with less armature current atreduced load.

The armature current limiting and armature voltage limiting features ofthe Fig. 3 modification are identical with the same features of the Fig.1 and Fig. 2 modifications.

Although in accordance with the provisions of the patent statutes thisinvention is described as embodied in concrete form and the principlethereof has been explained together with the best A further advantage l6mode in which it is now contemplated applying that principle, it will beunderstood that the apparatus shown and described is merely illustrativeand that the invention is not limited thereto, since alterations andmodifications will readily suggest themselves to persons skilled in theart without said armature current substantially constant, ad-

ditional electric valve means for controlling the supply of current tothe field winding of said motor, and means for maintaining an operatingcharacteristic of said motor substantially constant at a predeterminedvalue comprising a sec- 0nd source of reference voltage, meansresponsive to said operating characteristic of said motor for producinga second signal voltage and means responsive to a change in thedifference of said second reference voltage and said second signalvoltage for controlling said additional electric valve means tocounteract said change.

2. A control system for an electric motor comprising electric valvemeans for controlling the supply of current to the armature of anelectric motor, a source of adjustable reference voltage, meansresponsive to the armature current for producing a signal voltageproportional to armature current, means responsive to the difference ofsaid signal voltage and reference voltage for controlling said electricvalve means to maintain said armature current substantially constant ata predetermined value that is dependent upon the magnitude of saidreference voltage, a second electric valve means for controlling thesupply of current to the field winding of the motor in one direction, athird electric valve means for controlling the supply of reverse currentto said field winding, a second adjustable source of reference voltagefor selectively controlling said second and third electric valve means,means for producing a second signal voltage proportional to themagnitude of an operating characteristic of said motor for c ope t gwith said second reference voltage to control said second and thirdelectric valve means selectively to maintain said operatingcharacteristic substantially constant at a value corresponding to themagnitude of said second reference voltage.

3. A control apparatus for an electric motor comprising electric valvemeans for controlling the supply of current to the armature, regulatingmeans responsive to variations of the armature current from apredetermined value for controlling said electric valve means tomaintain said armature current substantially constant at a predeterminedvalue, a second electric valve means for controlling the supply ofcurrent in one direction to the field winding of the motor, a thirdelectric valve means for controlling the supply of reverse current tothe field winding, a source of adjustablereference voltage forpresetting the speed and direction of rotation of the motor, means forproducing a signal voltage proportional to the speed of the motor, andmeans responsive to the magnitude of y 17 said reference voltage forselectively energizing said second and third valve means and responsiveto the difference of said signal and reference voltages for controllingthe energized valve means to maintain the speed of the motorsubstantially constant at a value corresponding to the magnitude of saidreference voltage.

4. A control apparatus for an electric motor comprising electric valvemeans for controlling the supply of current to the armature of themotor, regulating means responsive to variations in the armature currentfor controlling said electric valve means to maintain said currentsubstantially constant at a predetermined value, a second electric valvemeans for controlling the supply of current to the field winding of themotor, a source of reference voltage, means for producing a signalvoltage proportional to the speed of the motor, means responsive to thedifierence of said signal and reference voltages for controlling saidsecond electric valve means to change the speed of said motor to a speedcorresponding to the magnitude of said reference voltage, and additionalelectric valve means responsive to a predetermined difference of saidsignal and reference voltages for rendering said armature currentregulating means temporarily inactive and controlling said firstelectric valve means to increase said armature current substantiallyabove said predetermined value of armature current thereby to forcesaidspeed change.

5. Control apparatus for an electric motor comprising electric valveapparatus for controlling the supply of current to the armature of themotor, regulating means responsive to variations of the armature currentfrom a predeter- 'mined value for controlling said electric valve ofcurrent to the field winding of the motor, a 4

source of adjustable reference voltage, a tachometer generator driven bythe motor for producing a signal voltage proportional to motor speed,means responsive to the difierence of said reference and signal voltagesfor controlling said second valve means to effect a change in the speedof said motor, electric valve means connected to by-pass said regulatingmeans and responsive to a predetermined change in said reference voltagefor controlling said first valve means to increase said armature currentabove said predetermined value to force said speed change, andadditional electric valve means responsive to said armature current forcontrolling said by-passing means to limit said armature to a valuesubstantially in,

. a reverse current to said field winding, a source of adjustablereference voltage, means for producing a signal voltage proportional tothe speed of the motor, means responsive to the magnitude of saidreference voltage for selectively energizingsaid second and third valvemeans to determine the direction of rotation of said motor andresponsive to a predetermined change in the difference of said referencevoltage and signal voltage i'or controlling said second and third valvemeans to effect a change in the speed of said motor, a pair of electricvalves selectively energized in response to the magnitude of saidreference voltage and responsive to a predetermined difference of saidreference voltage and signal voltage to control said first valve meansto increase said armature current above said predetermined value toforce the speed change of the motor, and means responsive to saidarmature current for controlling said pair of electric valves to limitsaid armature current to a value substantially in excess of saidpredetermined value of armature current.

7. Control apparatus for an electric motor comprising electric valveapparatus for controlling the supply of current to the armature of themotor, regulating means responsive to variations of the armature currentfrom a predetermined value for controlling said electric valve means tomaintain said current substantially constant at said predeterminedvalue, a second electric valve means for controlling the supply ofcurrent to the field winding of the motor, a source of adjustablereference voltage, means for producing a signal voltage proportional tothe speed of said motor, means responsive to the difference of saidreference voltage and signal voltage for controlling said second-valvemeans to effect a change in the speed of the motor, a first controlelectric valve having an input circuit including a control gridconnected to be responsive to the diiference of said reference voltageand said signal voltage and having an output circuit connected tocontrol said first electric valve means to increase said armature toforce said speed change in response to a difference of said referenceand signal voltages of a predetermined value, and an additional controlelectric valve responsive to said armature current and connected to saidcontrol grid for controlling said first control valve to limit saidarmature current to a value substantially in excess of saidpredetermined value of armature current.

8. Control apparatus for an electric motor comprising electric valvemeans for controlling the supply of current to the armature of themotor, regulating means responsive to variations in the armature currentfrom a predetermined value for controlling said electric valve means tomaintain said armature current substantially constant, a second electricvalve means for controlling the supply of current to the field windingof the motor, a source of adjustable reference voltage, means forproducing a signal voltage proportional to the speed of the motor, meansresponsive to the diiference of said reference and signal voltages forcontrolling said second electric valve means to effect a change in thespeed of the motor, electric valve means connected to by-pass saidregulating means and responsive to a predetermined change in thedifference of said reference voltage and signal voltage to control saidfirst valve means to increase said armature to force said speed change,and means responsive to the voltage supplied to the armature of saidmotor for controlling one of said electric valve means to limit thearmature voltage to a predetermined value.

9. Control apparatus for an electric motor comprising electric valvemeans for controlling the supply of current to the armature of themotor, regulating means responsive to variations in the armature currentfrom a predetermined value for controlling said electric valve means tomaintain said armature current substantially constant, a source ofadjustable reference voltage, means for producing a signal voltageproportional to the speed of the motor, a second electric valve meanshaving an input circuit connected to be responsive to the difference ofsaid reference and signal voltages and an output circuit for controllingthe supply of current to the field winding of the motor electric valvemeans responsive to a predetermined change in the difference of saidreference voltage and signal voltage to supersede said regulating meansand control said first valve means to increase the current supplied tosaid armature above said predetermined value, and electric valve meansconnected to said input circult and connected to be responsive to thevoltage supplied to said armature for controlling said second electricvalve means to limit said voltage to a predetermined value.

10. Control apparatus for an electric motor comprising a first electricvalve means for controlling the supply of current to the armature of themotor, a source of adjustable reference voltage, means for producing asignal voltage,

a second electric valve means for controlling the supply of current tothe field winding of the motor, a third electric valve means forcontrolling the supply of reverse current to the field winding, meanscontrolled by said reference voltage for selectively energizing saidsecond and third electric valve means and responsive to the differenceof said signal and reference voltages for controlling the energizedvalve means to vary the torque of the motor, and additional electricvalve means responsive to a predetermined difference of said referenceand signal voltages for controlling said first valve means to vary thecurrent supplied to the armature of the motor.

11. Control apparatu for an electric motor comprising a first electricvalve means for controlling the supply of current to the armature of themotor, a source of adjustable reference voltage, means for producing asignal voltage, a second electric valve means for controlling the supplyof current to the field winding of the motor, a third electric valvemeans for controlling the supply of a reverse current to the fieldwinding, a pair of switching electric valves selectively controlled inresponse to said reference voltage for selectively energizing saidsecond and third valve means and responsive to the difference of saidreference and signal voltages for controlling the selected valve meansto vary the field current, a pair of control electric valves selectivelycontrolled in response to said reference voltage and responsive to thedifference of said reference and signal voltages for controlling saidfirst electric valve means to vary the armature current supplied to saidmotor.

12. A control apparatus for an electric motor comprising an electricvalve rectifier for controlling the supply of current to the armature'ofthe motor, a source of adjustable reference voltage, means for producinga signal voltage, electric valve means for controlling the supply ofcurrent to the field winding of the motor. a second electric valve,means for controlling the supply of a reverse current to the fieldwinding, a pair of switching electric valves oppositely responsive tosaid reference voltage for selectively energizing said first and secondvalve means selectively to control the direction of the torque of themotor and responsive to the difference of said reference and signalvoltages to vary the magnitude of the torque, and a pair of controlelectric valves selectively controlled in response to the polarity ofthe difference of said reference and signal voltages and responsive tothe magnitude of said difference voltage for varying the conductivity ofsaid rectifier.

13. A control apparatus for an electric motor comprising an electricvalve rectifier for controlling the supply of current to the armature ofthe motor, a.source of adjustable reference voltage, means for producinga signal voltage, electric valve means for controlling the supply ofcurrent to the field winding of the motor, a second of said differencevoltage for varying the conductivity of said rectifier, means responsiveto the current supplied to the armature for controlling said controlvalves to limit said current to a predetermined value.

14. Control'apparatus for an electric motor comprising an electric valverectifier provided with a control electrode for controlling the supplyof current to the armature of the motor, a source of adjustablereference voltage, means for producing a signal voltage, a firstelectric valve means for controlling the supply of current to the fieldwinding of the motor, a second electric valve means for controlling thesupply of a reverse current to the field of the motor, a pair ofswitching electric valves oppositely responsive to the polarity of thedifference of said reference and signal voltages for selectivelyenergizing said first and second valve means to determine the directionof the torque of the motor and responsive to the magnitude of saiddifference voltage for varying the conductivity of the energized valvemeans to vary the torque, a first control electric valve responsive toone polarity of said reference voltage for varying the conductivity ofaid rectifier in accordance with the magnitude of said difierencevoltage, a sense inverter valve responsive to the opposite polarity ofsaid difference voltage and a second control valve controlled therebyfor controlling the conductivity of said rectifier.

15. Control apparatus for an electric motor comprising an electric valverectifier for controlling the supply of current to the armature of themotor, a source of adjustable reference voltage, means for producing asignal voltage, a first electric valve means for controlling the supplyof current to the field winding, a second electric valve means forcontrolling the supply of a reverse current to the field winding, a pairof switching electric valves oppositely responsive to the polarity ofthe difference of said reference and signal voltages for selectivelyenergizing said first and second electric valve means and responsive tothe magnitude of said difference voltage for controlling the energizedvalve means to vary the current supplied to the field of the motor,

21 and a pair of control electric valves selectively controlled by saidswitching valves for controlling said rectifier to vary the currentsupplied to the armature of the motor.

16. Control apparatus for an electric motor comprising an electric valverectifier for controlling the supply of current to the armature of themotor, a source of adjustable reference voltage, means for producing asignal voltage, a first electric valve means for controlling the supplyof current to the field winding, a second electric valve means forcontrolling the supply of a reverse current to the field winding, a pairof switching electric valves oppositely responsive to the polarity ofthe difference of said reference and signal voltages for selectivelyenergizing said first and second electric valve means and responsive tothe magnitude of said difference voltage for controlling the energizedvalve means to vary the current supplied to the field of the motor, apair of control electric valves selectively controlled by said switchingvalves for controlling said rectifier to vary the current supplied tothe armature of the motor, and means responsive to the armature currentfor controlling said rectifier to limit the armature current to apredetermined value.

17. Control apparatus for an electric motor comprising an electric valverectifier for controlling the supply of current to the armature of themotor, a source of adjustable reference voltage, means for producing asignal voltage, a first electric valve means for controlling the supplyof current to the field winding, a second electric valve means forcontrolling the supply of a reverse current to the field winding, a pairof switching electric valves oppositely responsive to the polarity ofthe difference of said reference and signal voltages for selectivelyenergizing said first and second electric valve means and responsive tothe magnitude of said diiference voltage for controlling the energizedvalve means to vary the current supplied to the field of the motor, apair of control electric valves selectively controlled by said switchingvalves for controlling said rectifier to vary the current supplied tothe armature of the motor, means responsive to the armature current forcontrolling said rectifier to limit the armature current to a.

predetermined value, and means responsive to the voltage supplied to thearmature for controlling said first and second electric valve means tolimit the armature voltage to a predetermined value.

18. Control apparatus for an electric motor subject to overhauling loadscomprising an electric valve rectifier for controlling the supply ofcurrent to the armature of the motor, a source of adjustable referencevoltage, a, tachometer generator driven by the motor for producing asignal voltage varying with the speed of the motor, a first electricvalve means for controlling the supply of current to the field winding,a second electric valve means for controlling the supply of reversecurrent to the field winding, means responsive to the polarity of thedifference of said signal and reference voltages for energizing saidfirst valve means to cause said motor to produce a motoring torque andresponsive to a reversal of said polarity in response to an overhaulingload for deenergizing said first valve means and energizing said secondvalve means to cause said motor to develop a braking torque, a pair ofcontrol electric valves selectively controlled in response to thepolarity of said difierence volt- 22 age and responsive to the magnitudeof said difference voltage for controlling said rectifier, and meansresponsive to said armature current for controlling said rectifier tolimit said current.

19. A control apparatus for an electric motor comprising means forcontrolling the supply of current to the armature of the motor,regulating means responsive to variations in the armature current forcontrolling said supply means to maintain said current substantiallyconstant at a predetermined value, a source of reference voltage, meansfor producing a signal voltage proportional to the speed of the motor,means for controlling the supply of current to the field winding of themotor comprising electric valve means provided with an input controlcircuit connected to be responsive to the difference of said voltages tomaintain the speed of said motor substantially at a value correspondingto the magnitude of said reference voltage and an electrical feedbackcircuit from said field winding to said input circuit for controllingthe operation of said electric valve means to modify the speed torquecharacteristic of said motor.

20. Control apparatus for an electric motor comprising means forcontrolling the supply of current to the armature of the motor, a sourceof adjustable reference voltage, means for producing a signal voltage,means for varying the torque of the motor comprising electric valvemeans provided with an output circuit connected to said field windingand a control grid circuit connected to be responsive to the differenceof said signal and reference voltages, and an electrical feedbackcircuit from said field winding to said control grid circuit formodifying said variation in torque comprising electrical conductors fromsaid field winding to said input circuit for supplying a voltage to saidinput circuit proportional to the voltage across said field winding.

21. A control apparatus for an electric motor comprising an electricvalve rectifier for controlling the supply of current to the armature ofthe motor, a source of adjustable reference voltage, means for producinga signal voltage, electric valve means for controlling the supply ofcurrent to the field winding of the motor, a.

second electric valve, meansfor controlling the supply of a reversecurrent to the field winding, a pair of switching electric valvesoppositely responsive to said reference voltage for selectivelyenergizing said first and second valve means selectively to control thedirection of the torque of the motor and responsive to the difl'erenceof said reference and signal voltages to vary the magnitude of thetorque, said switching valves having input and output circuits anelectrical feedback circuit from said field winding to one of saidswitching valves for' modifying the speed torque characteristic of themotor comprising electrical connections from said field winding to saidinput circuit for supplying to said input circuit a, voltageproportional to the voltage across said field winding, and a pair ofcontrol electric valves selectively controlled in response to thepolarity of the difference of said reference and signal voltages andresponsive to the magnitude or said difference voltage for varying theconductivity of said rectifier.

22. A control system for an electric motor comprising electric valvemeans for controlling the supply of current to the armature, meansresponsive to variations from a predetermined value in the armaturecurrent of said machine for controlling said electric valve means torestore said winding to said additional electric valve means formodifying said operating characteristic com- Number prising apotentiometer connected across said 15 2,400,599

24 field winding for deriving a portion of the voltage across saidwinding and electrical connections from said potentiometer to said inputcircuit for supplying said derived portion of said voltage 5 thereto.

ORRIN W. LIVINGSTON.

REFERENCES CITED The following references are of record in the tile ofthis patent:

UNITED STATES PATENTS Name Date Reeves May 21, 1948

